Grid-leak biasing is a technique involved in vacuum tube electronics for biasing the signal applied to the signal grid, by superposing the signal on a direct voltage derived from the signal itself. The direct voltage for biasing the grid is developed by peaks of an input signal, which is resistance-coupled to the grid, being clamped by grid current conduction, as the grid-to-cathode rectifier is forward biased. This charges the dc-blocking capacitor in the resistance coupling network, operating it as a dc restoration capacitor to apply reverse bias to the vacuum tube between signal peaks. Grid-leak biasing is a form of self-biasing usually used to bias a low- or moderate-transconductance device into a linear operating region.
Self-biasing can be effective with a voltage-controlled device, since its essentially open-circuit input impedance does not demand such high discharge current from the dc-blocking capacitor as to cause severe droop in the dc restoration. Self-biasing is generally not employed with a current-controlled device, because it usually has a relatively low input impedance at its input electrode. This low impedance tends to discharge the dc-blocking capacitor too rapidly. Self-biasing as known in the prior art requires a depletion-mode operation characteristic in the device. So, self-biasing is not employed with bipolar transistors.
Self-biasing can be used with a junction-gate field effect transistor, which like a vacuum tube is invariably a depletion-mode device. Self-biasing can be achieved with insulated gate field effect transistors of depletion-mode type, even though they do not have gate-to-source rectifiers, by connecting separate rectifier devices between their gate and source electrodes.
Enhancement-mode insulated-gate field effect transistors require forward static bias potential, rather than reverse static bias potential, applied between source and gate electrodes to place them in their linear amplification regions. So one's natural inclination is to regard them as unsuitable amplifier devices for self biasing. It would be even more desirable to develop, forward self bias in an enhancement mode amplifier device than reverse self bias in a depletion mode amplifier device. With such self bias the enhancement-mode amplifier device would consume no more quiescent power than necessary for amplifying an input signal. As the input signal swing increases on average, self bias would further forward bias the amplifier device input circuit to avoid excessive clipping; but as the input signal swing decreases on average the amplifier device input circuit would be less forward biased, to reduce quiescent consumption in the device.